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Compass, Magnetic, Type B-20 Vertical Card, WWII, US Army Air Force


Compass, Magnetic, Type B-20 Vertical Card, WWII, US Army Air Force


Here is a WWII-era Type B-20 Direct Reading Magnetic Compass, made by Autoflight, part number 2221.  The B-20 Compass design is unique in that it contains a magnetic horizontal float suspended in damping fluid that engages the directional pointer on a vertical compass card to display direction. The pointer indicates the direction of aircraft flight. The knob in the lower left corner manually rotates the course indicating pointer (i.e., the two parallel arrows) to the desired course.  When the compass pointer is centered within the course indicating pointers, the aircraft is headed in the desired direction.  (e.g., in the main photo, the course is set at 2 degrees N and the aircraft is traveling ENE at 59 degrees).

The interface design is identical to the Remote Compass Indicator AN5730-2A, which displayed compass direction received from electrical signals from a magnetic compass transmitter typically located in the wing of the aircraft away from local magnetic disturbances caused by engines, turrets, etc.

The B-20 Compass had a small module containing magnets at the top of the compass to compensate for deviation.  It was adjusted by removing the small adjustment screwdriver at the top, which opened a panel revealing the adjustment screws for N-S or E-W compensation (see close-up photo).

Our example is in excellent condition given its age. The serial number prefix is AF-43, evidence of a 1943 manufacturing year for an order from the US Army Air Force.  The fluid is low by about a teaspoon, as evidence by the fluid line across the face between 330 and 30 degrees (see photo). The compass pointer is slightly sticky as it rotates, and the course knob is a little stiff but turns 360 degrees. The glass and case are intact and the markings are in perfect condition.

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